Blown film lines are known. The lines are fed plastics in granulated form, which are then plasticized into a viscous mass under high pressure in extruders. This mass is formed into an annular shape in a die head and exits the die head through an annular nozzle. The mass already forms a film tube from the time of leaving the annular nozzle. The film tube is taken off towards the top along a tube-forming zone, in which compressed air is injected into the interior of the film tube. This results in transverse stretching of the film tube. At a tolerable distance from the annular nozzle, cooling the melt is achieved by using an active means of cooling for the rising film tube. On its path to the top, the film tube—in a mostly semi-crystalline state—passes through a calibration cage and then a collapsing frame, which flattens the tube. The collapsing frame unit feeds the double-layer film web to a pre-nip system. The pre-nip system usually includes a pair of rolls through the gap of which—the nip—the film runs. The pre-nip system is followed by a nip system, in which the film tube is finally turned into a practically air-free double-layer film web. From this point in the process on at the latest, or rather, already from the pre-nip process on, there is a double-layer film web. The distance from the pre-nip system to the nip system has been sized such that the film can dissipate the heat from the extrusion process during the transport between the two roll pairs. Thus, in this way, the film tube is cooled additionally so that it can then be processed further. This includes, e.g., the slitting of the film tube, resulting in two separate film webs.
Many lines work without a pre-nip system; instead, they route the film directly from the collapsing frame to the nip system. In such a line design, the rising film is also already cooled off so much at the nip roll pair that strong intervention at the surface will not do any, or little, damage to the film. For the nip system normally draws the film up at a significantly higher speed than the speed with which the film is extruded at the annular nozzle. An exemplary speed ratio is from about 10:1 up to 20:1. The blown film is immediately inflated with compressed air from the inside as the tube forms above the annular nozzle, and thus stretched transversely. At the same time, the nip roll pair draws the film off upwards at high speed so that longitudinal stretching occurs below the frost line.
Thus, in total, the film tube is stretched biaxially below the frost line. Depending on the application the film end product will be used in, the share of longitudinal stretching or that of lateral stretching can prevail.
However, a blown film line will always have to grapple with the technical disadvantage that the visual film quality cannot keep up with the film quality of cast films. This results from the fact that the rising film tube shape cools off relatively slowly. The longer the cooling process of the plastic melt takes, the more opaque and less glossy the film surface will get.
However, in order to be able to exert a sufficient force onto the rising film with the nip roll pair, the film must have cooled off relatively strongly. Given the slow cooling-off speed of the extruded film, this results in great built height for blown film lines. Thus, as fast as possible above the nip system, the double-layer film web is deflected horizontally, routed next to the line and downward from there for the further treatment steps. Usually, a winder is placed next to the system on the floor of its erection location, which winder winds the double-layer film web onto a roll for further transport.
Upstream from the winder, stretching systems are sometimes provided, whereby the term “stretching” is understood as a generic term for “strong stretching in machine direction, MDO” and “extending in machine direction” in the context of the present patent application.
An “MDO stretching” system stretches the film by more than 5%, at any rate, in the longitudinal/machine direction, preferably by 50% and more, often also by up to 1,000%. Such systems are often called “MDO”, which stands for “machine direction orientation”; i.e., for orienting the plastic molecules in the direction of the machine; i.e., the material's transport direction through the line.
As an alternative to an MDO, a so-called layflat package can be provided as a stretching system upstream from the winder. This package irreversibly “stretches” the film, usually by between 0.5% and 5% in the direction of the machine, which merely serves to even out differences in running lengths over the width of the double-layer film web and in the directional stability of the film web, allowing the film to be more easily wound up and processed further.
Both stretching systems—i.e., an MDO and a layflat package—are easily technologically comparable insofar as they stretch the film longitudinally. For this purpose, after an initial slower roll, or, e.g., after additional passive rolls, a roll that is driven faster follows immediately. The speed differential between the two rolls, which can also be embodied as nip roll pairs, which each transport the film by means of adhesive friction, results in a change of the film's length.
The distance between the two areas in which the film is transported at the circumferential speed of each roll is called the “stretching section”, or as “stretched length”, when projected onto the machine direction.
In a roughly central section of the film web's enveloping of a roll, the film web is transported by means of adhesive friction, i.e., at the circumferential speed of the roll. Static friction ends even before the film web leaves the roll surface at a lift-off point. This is of significance particularly when the subsequent roll runs at a higher circumferential speed; i.e., the film web transitions already on the roll surface from adhesive friction to the faster dynamic friction and only then lifts off from the roll surface.
The same principle can also be found in the winding process onto a roll: The film web already makes contact with the revolving roll surface from a contact point; adhesive friction, however, does not begin until beyond the contact point.
For simplicity's sake, the term “points” is used here. A film web lifts off a roll at a lift-off line and lands on a roll surface with a contact line. In a side view, the two-dimensional film web has, however, been reduced to a line by one dimension; accordingly, the lift-off and the contact line are each reduced to points by one dimension.
It should be noted that instead of a roll, as a rule, a nip roll pair can also be used just as well for transporting the film. For simplicity's sake, the present application usually speaks only of a roll, but it thereby also means a nip roll pair as a replacement means known in the art.
A nip roll pair can tend to effect more secure gripping of the film because the film surface is gripped from both sides. However, a roll impinging laterally can also exert a sufficient longitudinal force on the film, which, e.g., depends on the surface design of the roll in concert with the respective film to be processed and, e.g., from the roll's arc of contact. Usually, for a single driven roll, a contact roll will be provided at any rate in order to ensure more reliably that the film can actually be gripped securely by the driven roll, excluding any slippage.
The blown film method is suitable for the manufacture of stretch plastic films. These films are stretched monoaxially in the direction of the machine in MD stretching systems, which results in films with reduced film gauges. MD stretching improves, e.g., the following film properties: Tear strength, rigidity, transparency, barrier properties, and/or machinability. The films are used, e.g., in flexible packaging.
In the manufacture of tube films, film gauge profile control systems with segmented control zones are used. These systems allow controlling the film gauge profile in such a manner that the thickness variances over the entire tube circumference are minimized. DE 100 47 836 A1 describes such a method for controlling the film gauge profile in blown film lines, specifically based on measuring one or several individual layer thickness(es) of a multi-layer film as a reference parameter.
Systems for controlling film gauges in longitudinal/MD stretching systems for cast films or laminated films are also known. Control is significantly easier as the film will not be reversed and thus, the direct assignment of the individual measuring points in the gauge profile diameter of the longitudinally stretched film to the extrusion nozzle including actuators, or respectively, to the control zones is possible.
From DE 39 41 185 A1, a method for controlling the film gauge of tube films from blown film lines with downstream axial or biaxial stretching of the inflated tube films in an oven is known, resulting in a final film with minimized gauge variances.
In the longitudinal stretching process of an MD system, the film is stretched in the direction of the machine according to the MD stretch degree, thus reducing the film gauge. At the same time, the film shrinks in the lateral direction, reducing the film's width. This shrinkage results in the stretched film becoming slightly thicker from the center of the film towards the film edges, despite the fact that it was controlled to have the most constant gauge possible during the preceding blowing process. This increase in film gauge is especially pronounced in the film edge areas. During the subsequent winding of the film, this causes thicker edges in the film rolls. With increasing roll diameters, the film web edges will increasingly be stretched, resulting in severe disadvantages for subsequent processing such as printing or laminating.
Shrinkage and thus, thicker edges on the film roll, can be reduced by measures such as a minimized stretching gap, suitable roll coating, mechanical or electro-static holding of film edges, optimized temperature management, or suitable selection of plastic materials. However, this is not sufficient for many subsequent processing steps. Only if the film edges are trimmed will the remaining film web have a sufficiently small variance from the gauge profile that is required for the subsequent winding of the film web and its further processing. But much of the film width is lost by trimming. Regardless of film width, about 200 mm are trimmed on either side of the film.
In WO 2014/023282 A1, published later by the same Applicant as the present one, it is proposed that the film be heated above the nip and then treated mechanically. Thus, the film can be brought to an easily processable temperature level from the initial heat, using little energy. According to a second aspect, it is proposed that a tensile force bottom-out brake be provided.
EP 2 277 681 A1, which is part of the prior art, discloses a control method for achieving a film having the most even gauge possible at the winder.
EP 1 147 877 A2 discloses in its second embodiment in FIG. 5 a manufacturing line for stretch film whereby, however, the initial heat is no longer present in the stretching section between the stretch roll pairs because there, the double-layer film web is routed around the take-off roll pair to the side and is not stretched until significantly later. In particular, there is little or no rising initial heat from the blown film process.
The same applies to U.S. Pat. No. 6,413,346 B.
U.S. Pat. No. 2,976,567 shows a cast film line, not a blown film line. Thus, there is no take-off roll pair; besides there is precisely no effect of great heat collecting above the die head, and thus, above the nip roll pair.
In U.S. Pat. No. 7,396,498 B1, optional stretching of the double-layer film web is performed at the very bottom directly next to the die head; i.e., practically on the factory floor.
U.S. Pat. No. 5,458,841 performs longitudinal stretching between a pre-takeoff roll pair and a takeoff roll pair; namely, above the rising blown film. But this is precisely where no heating is provided for the film; instead, the citation calls the stretching section “cold orientation zone”. Besides, above the ultimate takeoff roll pair, no more mechanical processing takes place. Instead, a reversing roll is used for immediate routing into the horizontal orientation and ultimately, further down.
In DT 1 504 461, an internal mandrel for heating the film tube is provided. The first takeoff roll pair does not close. Stretching downstream from the first takeoff roll pair is performed by means of positive pressure flowing through.
In AT 267 160, the blown film is embossed within the take-off roll pair.
AT 342 292 provides for the tube film to be routed through a number of infrared heaters that increase the tube film temperature to the temperature required for stretching. The tube film is then stretched in the transverse direction from the extrusion direction by introducing compressed air into it through a pipe, and it is simultaneously stretched in its longitudinal direction using means that are not shown, effecting an airtight sealing of the tube and taking it off at a speed that is greater than the speed with which it is being transported by the nip rolls. In the meaning of the preceding, the means for longitudinal stretching that are precisely not shown are the nip rolls, and beyond them, apparently no further treatment is to happen; not to mention that the citation extrudes from top to bottom and is thus anyway in a position where it cannot use the rising heat as well.
CH 432 815 also deals with the design of the line upstream from the takeoff roll pair; not, however, with, e.g., the design downstream from the takeoff roll pair.
The same applies to CH 475 082.
DE 21 32 259 C3 describes rather unrelated prior art.
DE 102 42 174 A1 explains a conventional blown film line, whereby the longitudinal stretch factor, or respectively, the blow-up factor is to be set by means of the ratio between the circumferential speed of the nip rolls and the internal pressure.
U.S. Pat. No. 6,447,278 B1 discloses lateral routing of the double-layer film web away, directly downstream of the takeoff roll pair.
U.S. Pat. No. 4,086,045 again shows a cast film line, which is therefore not highly relevant here because there is no processing above the rising heat from the extruder.
U.S. Pat. No. 3,768,949 shows an early embodiment of a reversing device, with the takeoff of the tube film being performed by two individual rolls that do not press against each other as a takeoff, but which, however, together represent a takeoff roll pair in the wider meaning.
U.S. Pat. No. 3,340,565 shows rotatable chill rolls for variably setting the cooling time.
U.S. Pat. No. 3,116,787 again shows a cast film line, which is therefore unrelated prior art due to a lack of processing steps above the hot extruder.
U.S. Pat. No. 4,676,728 provides for a reversing device with vertically standing reversal bars or rolls. The same applies to U.S. Pat. No. 5,727,723.
In DE 35 08 626 C1, rolls for threading the incoming film tube can be moved apart when the blown film line starts up. This is followed by a comb-like movement against each other of the reversing bars and the reversing rolls until an operating position has been reached.
In DE 692 08 002 T2 also, longitudinal stretching of the film tube is only performed upstream from a pre-takeoff roll; i.e., not downstream from the takeoff roll pair. Besides, there is no heating taking place in the cold orientation zone there.
In GB 2 201 371 A, a tube film is first unwound from a roll, then routed upwards above a blown film line, heated there, then routed vertically downward while being inflated, and finally taken off and wound up again. Heating downstream from the takeoff roll pair is not provided for, and besides, a hot die head is not provided for so that the heat generated above the latter does not exist, and thus cannot be used.
WO 2005/102666 A1 shows a blown film line in which either the clearance between the pre-takeoff roll pair and the takeoff roll pair is adjustable by means of a vertical adjustment mechanism, or in which a carousel with different rolls is provided, whereby in both cases, the double-layer film web is routed downstream from the takeoff roll pair, first laterally and then downward.
Above the treatment roll path, a reversing bar device can be provided for, in particular, within a reversing device. By means of reversing rotation of reversing bars and/or rolls, the reversing device provides for even laying of any point in the film tube having an uneven thickness, resulting in total in quite even winding on the roll. A reversing device can be seen, e.g., in EP 0 673 750 A1.
A reversing device shall, in the context of the present patent application, not be considered a “treatment roll path”.
A treatment roll path in addition also preferably comprises rolls exclusively; one could imagine also reversing bars or other means for guiding or rerouting film.
In addition, a reversing device does not include an active heating device for the double-layer film web.
The task underlying this disclosure is to provide an improvement of or an alternative to the prior art.